Re: gEDA-user: "soldermask via caps"

[Dietmar Schmunkamp <dietmar@xxxxxxxxxxxxxxx>]

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Am 21.05.2010 16:52, schrieb Stefan Salewski:
> On Fri, 2010-05-21 at 12:55 +0100, andrew whyte wrote:
>>
>>   There is a known problem in layouts with vias inside pads, where
>> solder wicks down into vias and causes the joint to be poorly
>> soldered.  I found this link  about how to avoid this problem:
> 
> I have absolutely no experience with this problem...
> I know that open vias in small pads can give problems -- solder can flow
> by capillary forces from the pad into the via.
> 
> I have never heard about this problem for large pads. For that case the
> amount of solder on the pad should be large compared to the volume of
> the vias, so there is no problem if some solder flows in the via hole.
> And you have a large area for electrical and thermal contact, so
> soldering should be not too critical. Capillary forces should move the
> fluid from width to narrow areas, so I do not really believe that all
> solder will propagate from the contact area of your device into large
> vias.
> 
> Just a guess of someone with no experience but physical background.
> 
> My next task, coming soon, is to solder a few of such thermal pads with
> a soldering iron: My plan: have some open 0.3 mm vias in the thermal
> pad, fill them with 0.2mm copper wire to improve thermal conductivity.
> Put some soldering past on the pad and via, mechanical fixate the device
> and heat the vias from backside with a really hot iron. Ok, that may
> fail, but gives me at least some experience...
> 
> Best regards
> 
> Stefan Salewski
> 
> 
> 
> 
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
> 

I tried this approach on my last board (the exposed pad of the IC I
choose is the only GND connection) and I failed miserably. My intention
was to reflow the paste from the bottom side via a via in the pad from
the bottom side of the board. It turned out that I used too much paste
and got shorts between the pins and the exposed pad. I finally asked for
professional help and will get a 2nd board back some time next week.
The people I tlaked to ensured me that without reflow your yield will be
close to 0 to solder an exposed pad from the reverse side.


- -- 

Mit freundlichen Gruessen

Dietmar Schmunkamp
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